Oil exploration involves evaluating reservoirs to determine the movement or absence of oil, gas, or water as the reservoir fluids are produced. Understanding movement of gas in reservoirs is important to the prevention of premature breakthroughs and optimization of reservoir performance. Gas movement in a reservoir can be monitored by gravity methods which include determination of borehole gravity and surface gravity of the reservoir. Borehole gravity data is used to map out the vertical distribution of oil and gas at a well and surface gravity is used to understand the surface distribution of gas.
Typically, borehole gravity surveys involve reading local earth gravity at a series of stations in a borehole. The difference in gravity (.DELTA.g) and the vertical distance (.DELTA.z) between two successive stations yield sufficient information to determine the bulk rock density of the strata adjacent the borehole. The bulk rock density is what gets mapped out to determine the vertical distribution of oil and gas as the reservoir fluids are produced.
Rock density, .rho., is given by the following expression: EQU .rho.=(F-.DELTA.g/.DELTA.z)/(4.pi.G)
where .DELTA.g/.DELTA.z is the vertical gradient of gravity between two spaced apart stations, F is the free air gravity, and G is the universal gravitational constant. The free air gravity, F, is typically determined during borehole gravity surveys, so that the only unknown is the rock density, .rho..
The further apart the station measurements are made, the deeper the zone of investigation. A 5 ft interval would produce a zone of investigation of 0 to 25 ft radial from the borehole. The deeper zone of investigation makes it possible to determine the true gas-oil contact, free from borehole effects such as localized gas cone, mud, and casing.
Gravity measurements are typically monitored in the microgal (10.sup.-6 cm/s.sup.2) or nano-g range to ensure useable data that provide an indication of untapped pockets of oil or gas in the strata adjacent a borehole. This level of resolution in gravity measurements requires a highly precise gravity sensor and carefully implemented measuring techniques. For instance, the gravity sensor must be oriented so that the sensitive axis of the sensor is vertically aligned. A deviation of the sensitive axis of the sensor by an angle .alpha. from the vertical corresponds to an error of g.(1-cos.alpha.), where g is the gravitational acceleration. Thus, a deviation by an angle a equal to 45 .mu.rad (or 0.00258.degree.) from the vertical would result in an error of about 1 microgal.
In addition to keeping the sensitive axis of the gravity sensor aligned with the vertical during gravity measurements, the depth measurements of the stations should also be accurate to within 1 mm to obtain a density with accuracy of 0.01 g/cm.sup.3.